The crisis at the top of the Labour government reached a new peak yesterday with allies of Tony Blair accusing supporters of Gordon Brown and leftwing MPs of seeking to stage a coup in which Mr Blair is before the summer forced to agree a timetable for leaving No 10.

Mr Blair's allies claim that he recently offered Mr Brown a fresh private reassurance on the date of the handover of power, including the circumstances of the transition, but the levels of distrust between the two are such that only a public promise to stand aside would assure the increasingly impatient chancellor. One Blair ally said the proposed date "was not soon enough" to satisfy the Brown camp.

In a day of fierce briefing and recriminations from both sides, it also emerged that a plan for a Brown-Blair initiative in the first week of January on Lords reform, party political financing and engaging local government, fell apart due to objections from Mr Brown about lack of preparation. "If that had gone ahead, we would not be in such a mess," said one official.

The Blairities also accuse Mr Brown of forming an unholy alliance with the enemies of New Labour on Labour's backbenches. They say Mr Brown's allies were plotting the move before the local elections, adding that this might, in retrospect, be seen as the weekend Labour lost the next election.

In an open assertion of his authority and determination to stay, Mr Blair sent out missions statements to ministers setting out their tasks up to 2009-10.

The new home secretary, John Reid, warned: "This isn't actually about a date, this is about forcing Tony Blair out. To force Tony Blair out, to stop the reform programme, to start to move back to old Labour ... would be a disaster."

Mr Brown was careful to distance himself from the backbenchers organising a letter calling on the prime minister to quit by the summer, and bring the party's national executive into the process. He said that would be a recipe for disaster, and again put the onus of a "stable and orderly" transition on Mr Blair

He said: "I think people will look to him, and it's not essentially a matter for me, it's a matter for him and the Labour party ... We don't need outriders dictating the agenda," reflecting his view that he must reach an agreement with No 10.

Mr Brown made it clear it was his task to help to develop the policies and party organisation for the future. He said that "in the next stage" of his political career he would ensure that Labour broadened its support base and was seen to be listening to the electorate. He said he did not know when Mr Blair would quit, and denied that he had been fully consulted about the cabinet reshuffle.

At least 50 backbenchers have agreed to sign, and if necessary, publish a letter calling on Mr Blair to agree a transition process by July. The letter says Labour's record is being overshadowed by "debilitating" speculation on Mr Blair's future. They want to see a "dignified, orderly and efficient" transition of power. It adds: "We therefore ask the NEC, in consultation with the prime minister, to lay out, no later than the end of the current parliamentary session, a clear timetable and procedure for the election of a new Labour party leader."

Stephen Byers, a leading Blairite, warned: "If we want to have an orderly transition, what we cannot have is the forced removal of Tony Blair as our leader. Those people who are organising a coup against him are playing a very dangerous game and they should stop."